The motion of articulated joints in vertebrates, and specifically in humans, may involve extremely complex motion between the bones forming the joint. Certain joints, such as the human shoulder and hip, involve ball and socket joints producing a three dimensional angular motion. Other human joints, such as the knee and elbow, produce significantly more complex motions involving both angular and linear motion.
The human knee, for example, produces a "polycentric" motion wherein the center of rotation of the tibia with respect to the femur varies as a function of the angle of rotation. The polycentric motion is caused by the complex relationship of the various interacting surfaces involved in articulation of the knee. Those relationships include a sliding of the tibia relative to the femur, and a rotation of the tibia relative to the femur. The motion of the tibia relative to the femur is further complicated by the three-dimensional component of motion outside the sagittal plane, and by the contribution of ligaments, tendons and cartilage structure.
As a results of contributions from all these factors, a given point on the tibia typically traces a complex, non-linear curve in the three-dimensional coordinate system of the femur. That curve is difficult to predict from static measurements of the various anatomical features contributing to the curve shape, because of the large number of those features and the difficulty in accessing them. It is therefore desirable to have a method for directly measuring the relative motion of the bones on either side of a joint in order to describe the motion of that joint.
An accurate representation of the motion of a joint may be used in many applications. For example, braces for use in the healing of damaged or misformed joints should be designed to closely imitate the measured motion of a healthy joint. In U.S. Pat. No. 4,782,842 to Fietti, Jr., a device for setting a fractured wrist in disclosed wherein metacarpal bones in the hand are restricted to a defined motion relative the radius in the forearm. The movement is controlled by a pinion engaged with an arcuate rack, which defines a compound motion of the wrist. The wrist is moved through a predetermined arc of flexion or extension imitating the motion of the healthy joint. The path of a healthy wrist is approximated by a series of arcs in the rack having varying radii and centers of curvature.
The measurement of polycentric motion may also be used as a diagnostic tool to detect problems affecting the motion of a joint. If the motion of a healthy joint is known, either by averaging the joint motion of a number of other healthy individuals, or by measuring the joint motion of the individual in question at an earlier time, the current joint motion may be compared to the healthy joint motion to diagnose problems in joint mechanics. Appropriate corrective action may then be taken.
It may be advantageous to measure the joint motions of a large number of individuals, in order to determine average joint motions across various populations. Such measurements may be made using the joints of either cadavers or of healthy individuals. There is therefore a need for a simple, systematic, repeatable apparatus and method for measuring the path of motion of a large number of joints.
Devices exist for the measurement of the angle of motion of a joint. For example, in U.S. Pat. No. 5,263,492 to Voyce, a recording goniometer for measuring the angle of joint movement of a knee is disclosed. The device includes two extension members which are strapped to the upper and lower leg respectively. The extension members are pivotally connected by a pivot pin. One of the extension members supports a pencil, while the other extension member supports a paper disk. The angle of travel during extension and flexion of the knee is recorded by the pencil on the paper disk.
U.S. Pat. No. 1,590,499 to Cozad discloses a measuring instrument for measuring the angle of rotation of an elbow or other joint. The device includes trough-shaped members that are strapped to the limb above and below the joint. The trough-shaped members are connected by pivot hinges on both sides of the joint. A protractor is rigidly attached to one of the trough-shaped members, while an indicating arrow having a center at the center of the protractor is attached to the other of the trough-shaped members. Angulation of the joint is read by observing the location of the indicator on the scale of the protractor.
None of the above disclose a method and apparatus for tracing a representation of the path of motion of a joint. The present invention is directed to such a method and apparatus.